Traditional retroreflective sheeting materials, such as those disclosed in U.S. Pat. Nos. 3,689,346, 3,712,706, and 3,810,804, the contents of which are incorporated herein by reference, are described as truncated solid corner-cube structures that are molded from tooling that comprises a plurality of element-forming cavities which produce corner-cube segments having substantially planar front major surfaces or truncated solid corner-cubes.
Traditional truncated corner-cube prisms have a base with three surfaces intercepting at an apex. As shown in FIG. 1, the prism is oriented such that an incident light ray 144 enters though the base 110, is reflected by the three corner-cube surfaces 112, and exits through the base 110, where it is emitted as retroreflected light ray 145.
Traditional full-square sided corner-cube retroreflective sheeting material, such as those of the types disclosed in U.S. Pat. Nos. 1,591,572, 1,807,350, 1,848,675, 2,055,298, 2,205,638, 3,069,721, 3,541,606, 3,833,285, 3,887,268, 3,894,790, 3,893,747, 3,894,786, 3,905,681, 3,923,378, 3,926,402, Re. 29396, 4,066,236, 4,066,331, 4,095,773, 6,015,214, 6,257,860, 6,318,978, 6,386,855, 6,447,878 and 6,533,887, the contents of which are incorporated herein by reference, are described as corner-cube structures that are molded from tooling that comprises a plurality of element-forming cavities which produce full-square-sided corner-cube segments. Traditional full square corner-cube prisms have a base with three surfaces intercepting at an apex. As shown in FIG. 2, the prisms are oriented such that the light ray 144 enters through the base 110 and is reflected by the three corner-cube surfaces 112. The base 110 may, for each individual full-square-sided corner cube, extend over a larger area than shown in FIG. 2. The retroreflected light ray is emitted as light ray 145 from the prism.
Assuming that the master tooling is diamond fly cut or machined and polished, solid truncated corner-cubes are produced from odd generation tooling, open-faced truncated corner-cubes are produced from even generation tooling and full-square-solid or open-faced corner-cubes are produced from either odd or even generation tooling. Sometimes “windage”, or compensation, is put into the dihedral angles of the tooling master because the polymer used to form the corner-cubes will shrink slightly during cooling or curing. In this case, with traditional solid or open-faced full-square-sided corner-cubes, proper selection of the correct generation tooling to form the corner-cubes is important.
FIGS. 3A and 3B are a sectional side view and top view respectively of a section of a conventional two-sided open-faced retroreflective sheeting including metallized, corner-cube surfaces 112A, 112B. In this example, the sheeting substrate 116 is relatively thin. A specular, optical, or reflective coating 120, such as aluminum, silver or gold, is formed on the corner-cube surfaces 112A, 112B of the substrate 116. Preferably, optical coating 120A, 120B is permanently attached to the corner-cube surfaces 112A, 112B, and is not easily removed or worn therefrom. If aluminum is used as the reflective coating, a vacuum-deposited protective over coat of SiO (Silicon Monoxide) or MgF2 (Magnesium Fluoride) is usually used to prevent the aluminum from oxidizing when exposed to air. In this manner, a first incident light ray R1, incident at a first surface 115A of the sheeting is retroreflected by the corner-cube surfaces 112A formed by the optical coating 120A on the first surface 115A of the sheeting, and a second incident light ray R2, incident at a second surface 115B of the sheeting is retroreflected by the corner-cube surfaces 112B formed by the optical coating 120B on the second surface 115B of the sheeting.
Conventional retroreflective sheeting of this type is made using a thin substrate material and has the disadvantage of requiring a relatively expensive specular reflective coating(s) on one or both sides of the part; otherwise the part will exhibit very little, for example, less than 0.1%, to no, retroreflection. Other types of two-sided retroreflectors are made with molded, transparent, either solid truncated or full-square-sided, corner-cube sheeting components that are assembled back-to-back, with the corner-cube apexes of one sheet facing the corner-cube apexes of the other sheet or back-to-back with an intermediate substrate layer between the corner-cube sheeting components. Such embodiments require complicated manufacturing techniques that add to product cost.